Diamante is fundamentally a summer town. July and August bring the crowds and the heat — temperatures regularly above 35°C, beaches packed, prices up. June and September are far better: warm enough to swim, with the sea at its highest temperature in September.
The first weekend of September brings the Festival del Peperoncino, five days of chili-themed everything that fills the town with visitors. If you want quiet, avoid it; if you want spectacle and serious eating, plan around it. By October the seasonal businesses close and Diamante reverts to a small fishing town, lovely but with limited options for dining and almost no nightlife.
Spring (April–May) is cool but pleasant for walking the murales without the heat. Winter is mild but mostly closed.